The Gram-negative bacterium Francisella tularensis is the causative agent of the zoonotic disease tularemia. Humans acquire tularemia from contact with infected tissues or materials, insect bites, consumption of contaminated food or water, or inhalation of aerosols. F. tularensis consists of 3 subspecies—tularensis, holarctica and mediasiatica—which differ in their geographic distributions and in their virulence in humans.
A vaccine against F. tularensis was developed a half-century ago, but it has not been approved for general use. This vaccine, called Live Vaccine Strain (LVS), is an attenuated form of Francisella tularensis subspecies holarctica, a much less virulent subspecies of F. tularensis than the highly virulent subspecies of concern as a bioterrorist agent, F. tularensis subspecies tularensis. The LVS vaccine is poorly characterized, unstable in that different colonial morphology types emerge on culture, and toxic to humans vaccinated with the LVS vaccine. Moreover, the LVS vaccine may not protect against the high doses of F. tularensis subspecies tularensis that might be released in an airborne bioterrorism attack.